1. Field of the Invention
The present invention relates to a composition for covering soil, and a method for applying said composition to a soil surface. The invention further relates to a method for using the composition to form a water-impervious cover for landscaping, construction and landfills.
2. Discussion of Background
Household, commercial and industrial solid wastes are commonly transported to sanitary landfills for disposal. Federal and state regulations require that the disposed solid wastes deposited in a landfill cell must be covered with a "daily cover"--typically 6 inches (about 15 cm) of clean, compacted soil--by the end of each day. A thicker "intermediate cover" consisting of 12 inches (about 30 cm) of clean, compacted soil is applied if time will elapse before additional wastes are dumped at the same location, and a "final cover" is applied when a landfill cell is to be closed. The soil layers serve to control disease vectors, fire hazards, odors, water infiltration, blowing litter, and scavenging by wild animals such as birds and rodents.
Not all soils are suitable for use as covers. Very sandy soils are subject to wind erosion and rain run-off, and promote excessive seepage. Clayey soils are difficult to get. They are also susceptible to root penetrations, which in turn provide channels for water infiltration. Regulatory requirements also allow the use of cover materials other than clays to replace or supplement local soils.
Current regulations convert landfills from "biological reactors" for the decay of organic wastes to dry, waste "storage facilities." Stringent design criteria for landfills consider factors such as the hydrogeologic characteristics of the facility and the surrounding land, the characteristics of the soil in the area, existing groundwater quality and usage, climate, and the expected volume and physical and chemical characteristics of the leachate. A landfill must be provided with a bottom liner that includes a layer of compacted soil having low hydraulic conductivity (typically no more than 1.times.10.sup.-7 cm/sec), leachate collection and removal systems, provisions for groundwater monitoring, and an approved plan for grading and final closure.
Closure requirements include a final cover system designed to minimize erosion of ground cover and water infiltration through the closed landfill. Typical state regulations require a low-permeability cover that includes a minimum of approximately 1 foot (about 0.3 m) of material that is capable of sustaining plant growth. The preferred final cover includes a type of soil that supports the growth of plants with well-developed root structures to help hold the soil and stabilize the landfill structure against erosion. The cover system must slope to promote positive drainage across the surface and prevent infiltration of surface water into the landfill, however, the slope must not be so great as to promote erosion. Slopes ranging from about 20.degree. to 35.degree. can be acceptable. Minimizing penetration of water into the landfill is important because surface water runoff can be disposed of to the environment, while rainwater that has percolated through a landfill is treated as leachate and must be treated as liquid waste, at potentially significant added expense.
A widely accepted and traditional method for closure is simply to cover the landfill with about 3 feet (about 0.9 m) of compacted clay. Many areas do not have readily available supplies of suitable clays. In addition, new regulations require a water-tight final cover in order to keep the entombed waste dry. Therefore, the clay cap has become inadequate due to roots from vegetation forming channels through the clay cover. However, equally effective materials may be substituted, and various synthetic materials have been suggested and are already being used for landfill closures. The preferred materials are polyvinyl chloride (PVC), very low density polyethylene (VLDPE), or similar pliable, substantially water-impermeable polymer membranes sandwiched between two layers of compacted soil. These membranes stop roots from penetrating into the waste. They allow the landfill operator to use available soils instead of buying clays, and also reduce rainwater infiltration into the stored waste.
These membranes have smooth, slippery surfaces, and, when embedded between layers of soil, tend to foster soil slippage as the angle of the slope increases. Various techniques are available to alleviate this problem, including reducing the surface slope of the landfill, terracing the slope, constructing embankments or ridges across the slope to intercept surface runoff, and embedding the film between two layers of nonwoven geotextile. Reducing the slope or terracing the sides of a landfill entails a substantial increase in cost and the use of large amounts of soil to achieve adequate closure. An ideal composition for use in landfills would be a geomembrane that is nonflammable, impermeable to water, and limits slippage of overburden.
There is a need for a nonflammable, water-impermeable composition that can be used to cover soil without causing undue slippage of the overburden and is simple and economical to manufacture.